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ABSTRACT

Preferred facies trends in ancient shallow-marine sediments have been determined by Markov-chain and substitutability analyses of detailed sections measured in Lower Cambrian outcrops in the Southern Canadian Rocky Mountains (St Piran Formation, 600 m of vertical section; Fort Mountain Formation, 500 m of vertical section), and in a Cretaceous subsurface reservoir in south-central Alberta (Viking Formation in the Garrington Field: 69 cores).

Those sections with more defined cycles are interpreted to result from progradation of shelf-ridge sands and gravels; examples with less defined order originated as inter-ridge deposits, in zones with complex palaeoflows and in areas of subdued palaeotopographic relief. The highest degree of facies organization (> 80% significant facies transitions) occurs in the Cretaceous Viking Formation sandstone and conglomerate ridges, interpreted to have had abundant sediment supply, water depths between fair- and storm-weather wave-base, and a strong littoral current system. A moderate degree of facies organization (70% significant facies transitions) occurs in the inter-ridge deposits of the Cambrian Fort Mountain Formation, interpreted to have had a variable sediment supply, water depths above storm-weather wavebase, and a variable, oscillating tidal current system. A poor degree of facies organization (< 45% significant facies transitions) occur in the ridge deposits of the Cambrian Fort Mountain and St Piran formations and in the inter-ridge deposits of the Cambrian St Piran and Cretaceous Viking formations. This facies disorganization occurs in sites where there was continuous and rapid subsidence (Cambrian) or in areas of subdued seafloor relief with complex current systems (Cretaceous). The pattern of facies relationships also varies. Relatively short linear patterns occur in all of the shelf-ridge deposits. This contrasts with the inter-ridge deposits in which there are long linear patterns or complex ring structures, interpreted to result from complex palaeoflow patterns in areas of subdued seafloor topography. Various statistical techniques (including Markov-chain and substitutability analyses) can aid the interpretation of very complex lithological successions, as shown by the shelf sandstones and conglomerates examined in this study.